Personally, I'd go for the red 'E' sweater. It's my favorite of his outfits. You could also get an angel hoodie and bring it with you - I don't really like the angel hoodie much in canon but this angel hoodie is hella cute. You would be able to wear it if people recognize it, or, not wear it if they do think it'd an angel cat thing. Or even just wear it for yourself on the daily, like, casually. Ooooh.

As for hair, I think what you have right now would look fine enough for Li'l E's current hairstyle, I mean, if you don't plan on getting a wig you would probably need to dye it though since his hair is red. Maybe a bang-trim would work wonders? But, I dunno, from what I see in that pic your hair looks fine enough as is. I suppose something like this would probably be most ideal. But of course, you can always just get a red wig and just cut it appropriately so that you don't have to mess with it. Depends on what's cheapest and most available to you, I suppose.

I know that the sweater is what he wore for awhile, but that was more pre-Lethe Lil'E, I don't know...
(With our current knowledge of Dman and Lil'E's relationship that comic huuurrrts.)

I hadn't thought of it, but I could always just wear the hoodie with the hood down. That way I don't look like a cat.

I do like the simplicity of his current default outfit though. I could just start off with this one and see how it goes, maybe.
Judging by this I could just get a semi-permanent colour to redden my hair a bit. Thank Tat it isn't bright red or something.

I could probably get a fringe cut (viva la commonwealth english) for it, but it would mean I couldn't do it on the other day of the con I plan to cosplay as Czes, because that character has a fairly long fringe. Aaargh but I want to promote Sinfest... idk I could do a test run either before or after and post photos. My main issue that I'd be covering myself in body paint, which is something generally best saved for special occasions. Even if thanks to the long sleeves and pants it's really only my face and hands. It would be amazing if I could get a friend of mine to do Pebbles though...